1. Field of the Invention (Technical Field)
This invention relates to the field of aerospace propulsion systems; more particularly, the invention relates to a liquid propellant rocket propulsion system design incorporating a pintle injector, radial injector plate and an ablative insert oriented to dampen resonant harmonic frequencies within the combustion chamber.
2. Background Art
Liquid propelled rocket engines are typically used to launch and guide orbital and sub-orbital vehicles and satellites. These engines typically include at least one injector element, a combustion chamber, a throat and a nozzle element. The injector element serves primarily to control the flow of the liquid propellants in order to achieve a desired mixture of fuel and oxidizer. Commonly these injectors accomplish an improved rate of combustion by impinging or atomizing the liquid propellants inside the combustion chamber, and provide efficient combustion and stability to the internal pressure of the combustion chamber.
As the liquid propellant is introduced into the combustion chamber and atomized, it is then ignited, creating a rapid expansion of gases which are emitted through the nozzle. The nozzle is designed to efficiently transfer this energy into a driving force, which propels the vehicle or satellite in the opposite direction of the exhaust. However, the forces created during the combustion cycle do not have to be time invariant and can oscillate. These forces can create pressure waves that resonate at a frequency that causes harmonic disturbance and instability within the combustion chamber. The harmonic resonance may exist in either the longitudinal, lateral or radial mode, or a combination of the three. The longitudinal resonance is often multiplied by the effect of the variable backpressure created at the injector, and thus causes a varying flow rate through the injector. This leads to varying thrust from the engine known as the “pogo” effect. These problems decrease the efficiency of the engine while increasing the likelihood of engine failure, and can also cause other performance related problems.
Prior art references have recently disclosed means for improving combustion stability. Some of these references disclose the use of baffles to reduce amplification of the resonant frequencies. One problem presented by these methods is the difficulty with replacing deteriorated parts due to their location near the head of the chamber. These methods frequently require the use of a coolant to prevent the baffles from becoming damaged during the combustion cycle, and do not provide phased damping methods to eliminate multiple orders of combustion instability.
Other problems include the loss of fuel within the chamber as it comes into contact with the combustion chamber walls. This liquid fuel does not atomize, and due to the high temperature within the chamber may boil and cause damage to the structure. This problem is further complicated as liquid oxygen or other cryogenic liquid injected in to the chamber is not properly impinged or vaporized, causing undesired combustion along the chamber wall and potentially creating further harmonic disturbance. This disturbance can actually decrease the rate of combustion by forcing the propellants in a direction away from the igniter. Although some engines disclose methods of distributing coolant along the outside of the chamber, either by a milled channel, platelet or tubular means, the outside of the chamber still reaches temperatures where the liquid oxygen or other liquid can cause damage and increase the risk of engine failure.
Other problems exist in the prior art relating to injection heads and distribution manifolds. Typical injector elements are machined to control the distribution of the liquid fuel, but often risk combustion instability in order to achieve efficiency. Some require numerous injector heads and complex tubing to distribute the liquid propellant evenly throughout the chamber. In addition, these types of machined injector plates present difficulties when scaling their manufacturing process due to the large number of machined injection points, and often require significant time for even minor design modifications. Thus, the traditional designs for liquid rocket engines have been very costly to produce and test in order to insure reliability. This cost is unacceptable for the Private Space Industry that must raise money on the open market rather than obtain government awarded development funds. The increased costs of testing and manufacturing often prohibit the ability for Private Space companies to provide services to private citizens.
Other systems teach a pintle injector to achieve combustion stability. By introducing the propellant from a center point in the chamber, the combustion cycle can be made more dynamically stable along the radial axis. However, there often remains a problem presented by instability along the longitudinal axis, which may be amplified by the presence of the pintle head inside the combustion chamber. Other methods of injecting propellants from a single pintle have been disclosed, but also fail to distribute the atomized gases evenly throughout the chamber. Another family of injector manifolds, referred to as coaxial pintle injectors, attempt to simplify the engineering and design process and reduce cost. In these designs both the oxidizer and the fuel is fed into the combustion chamber. The pintle often is designed with two sets of radial holes near its tip, one for oxidizer and one for fuel. These holes are designed to spray both the oxidizer and fuel streams into each other to achieve efficient mixing.
However, in practice the coaxial pintle design has proven to be expensive and problematic. Coaxial pintles are still sensitive to combustion instabilities, and the relative simplicity of the design leaves little to be changed in order to correct for this phenomena. In addition, the coaxial pintle is difficult to design and often requires extensive testing, which in turn drives up the cost of manufacturing.
Other methods have disclosed the use of an ablative material to insulate the chamber walls during the combustion cycle. Ablatives are also advantageous because they often eliminate the need for a complex cooling system and reduce the overall weight and complexity of the chamber. Typically, ablative chambers are comprised of an epoxy-resin or phenolic material, which during the combustion cycle vaporize and cool the adjacent chamber wall surface. As the material vaporizes it cools the chamber wall and leaves behind a layer of char, further insulating the chamber wall from the combustion elements and limiting deterioration. The particular thickness of the ablative wall depends on the heat generated during the combustion cycle and the internal pressure in the chamber. Therefore, many of the prior art references for ablative materials disclose varying profiles and thicknesses to meet the particular needs of their engine design. Many of these ablative chamber linings require significant time and effort to replace between launches. Therefore, a need also arises to provide an ablative material insert that provides ease of replacement and accounts for combustion instability along its contour.
It is therefore one object of the present invention to provide a unique combination spud and pintle injector, where the geometry of the injector is chosen in relation to the geometry of the combustion chamber to damp resonant frequencies.
It is another object of the invention to provide a combination spud and pintle injector which further provides spray intersection points in desired quantities and at desired locations to damp resonant frequencies within the combustion chamber.
It is another object of the invention to provide a radial injector plate surrounding the combination spud and pintle injector, where the geometry of the radial injector plate is chosen in relation to the geometry of the combustion chamber to damp resonant frequencies.
It is another object of the invention to provide a radial injector plate which further provides spray intersection points to damp resonant frequencies within the combustion chamber.
It is another object of the invention to provide a certain number of acoustic chambers within a radial injector plate which include igniters and independent injection elements, and which may be tuned to damp various resonant frequencies within the combustion chamber.
It is another object of the invention to provide a feedback control loop for damping axis oscillation detected within a certain number of acoustic chambers within a radial injector plate.
It is another object of the invention to provide a propellant accumulator external to the engine manifold, thus eliminating the requirement of a turbo pump or other device to provide propellant to the injectors, and which may be tuned to damp longitudinal resonant frequencies occurring in the engine manifold and the vehicle.
It is another object of the invention to provide an ablative insert to improve the stability of the combustion cycle, to protect the interior of the combustion chamber shell, and which is designed to reduce harmonic disturbance within the chamber.
These and other benefits of the present invention in its various embodiments will become apparent from the specification and appended claims.